In a Tokamak type plasma seal-off device, a diverter is used which is adapted to produce a magnetic line of force to direct the escaping plasma to an exhaust portion (pump) without hitting directly against the nearby wall and thus to reduce impurities in the plasma. During normal operating conditions or during disruption (phenomenon where the structure of plasma changes rapidly or disrupts sometimes), the diverter is subjected to harsh thermal stress, particle load and electromagnetic force. The diverter is fixed to a water-cooled cooling member to remove heat. The cooling member is supported on the inner wall of a vacuum container and insulated therefrom. One application of the insulating member according to the present invention is to provide insulation between the cooling member for supporting and cooling the diverter and the inner wall of the vacuum container.
Heretofore, earthenware, porcelain (insulator), bakelite, mica, plastics and rubber have been used as insulating members. Such an insulating member is placed between a substrate and a conductive material and they are brought together by fastening, bonding, soldering or fitting, to form an electric part. In normal conditions, such an electric part reveals desired insulation properties.
But if such an electric part is used in harsh conditions such as high-temperature, high-voltage and high-temperature-gradient in a limited space where it is subjected to impact load, e.g. in a vacuum container for high-temperature plasma, it is required to have high heat resistance, good insulation properties, and high thermal shock resistance and good shock resistance. It is required to have a dielectric strength of 1 KV or higher and a heat resistance of 600.degree. C. or higher. It is further required that it take up less space.
In order to meet all these requirements, it is difficult to use any of conventional insulating members for the following reasons:
1) Insulators made of earthenware or ceramics have to be fastened in position with bolts and nuts or the like. Thus, the entire device tends to be large in size. If bonded by brazing, such an insulator is liable to crack or peel due to a difference in thermal expansion coefficient between itself and the substrate metal. Further, since such an insulator has generally a low thermal conductivity, it cannot withstand a sharp temperature gradient. Moreover, due to its hardness and brittleness, it cannot be used at a location where impact load acts.
2) Bakelite, plastics and rubber have such low heat resistance that they cannot be used in any environment where the temperature exceeds 200.degree. C. continuously. Also, in a vacuum condition, gas discharge will be inevitable.
3) Mica has a high heat resistance and good insulating properties. But its strength is low due to its cleaving characteristics.